Gas concentration sensor drift and failure detection system

ABSTRACT

A system for detecting when a gas concentration sensor has drifted or failed is disclosed. The system has a first gas concentration sensor configured to detect a first gas concentration and generate a corresponding first signal. The system also has a second gas concentration sensor configured to detect a second gas concentration and generate a corresponding second signal. In addition, the system has a controller in communication with the first and second gas concentration sensors. Based on the first and second signals, the controller is configured to provide an out-of-tolerance gas concentration sensor warning.

TECHNICAL FIELD

The present disclosure relates generally to a detection system and, more particularly, to a system for detecting when a gas concentration sensor has drifted or failed.

BACKGROUND

Combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous and solid compounds, including particulate matter, nitrogen oxides (NO_(x)), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. To comply with these emission standards, engine manufacturers employ gas concentration sensors. But, gas concentration sensors are wear-out devices that eventually fail and require replacement (i.e. gas concentration sensors eventually become faulty). Furthermore, gas concentration sensors drift over time (i.e. measurements become miscalibrated). Miscalibrated measurements may result in noncompliance with the emission standards. Therefore, gas concentration sensors must be recalibrated periodically. Replacing and recalibrating gas concentration sensors is costly in both labor and parts, and this problem can be exacerbated when the engine is remotely located. Specifically, because reliable testing and calibration equipment may be unavailable, gas concentration sensors may be needlessly replaced.

One way to minimize the affect of a faulty gas concentration sensor is described in U.S. Patent Application Publication No. 2004/0221641 (the '641 publication) by Moritsugu et al., published on Nov. 11, 2004. The '641 publication describes a fault detecting apparatus for a gas concentration sensor. The fault detecting apparatus includes a storage device and a fault detecting circuit. The storage device stores conditions in which certain fault types may be detected. When one of these conditions is detected, the fault detecting circuit initiates detection of the corresponding fault type. Specifically, the fault detecting circuit determines whether there is a fault based on an output of the gas concentration sensor. If there is a fault, the fault detecting apparatus turns on a malfunction indicator lamp. Additionally, if the fault is minor, the fault detecting apparatus adjusts the functioning of the gas concentration sensor. But, if the fault is major, the fault detecting apparatus discontinues use of the gas concentration sensor.

Although the fault detecting apparatus of the '641 publication may improve detection of a faulty gas concentration sensor, it may do little to improve detection of a miscalibrated gas concentration sensor. Furthermore, though the fault detecting apparatus of the '641 publication may turn on a malfunction indicator lamp, it may do little to reduce the economic impact of the gas concentration sensor's failure. In particular, the operator may need to discontinue use of an affected engine until the gas concentration sensor can be replaced. In addition, the fault detecting apparatus of the '641 publication may increase the complexity and cost of the gas concentration sensor, but it may do little to prolong the life of the gas concentration sensor.

The disclosed system and method are directed to overcoming one or more of the problems set forth above.

SUMMARY

In one aspect, the present disclosure is directed to a drift and failure detection system for a gas concentration sensor. The system includes a first gas concentration sensor configured to detect a first gas concentration and generate a corresponding first signal. The system also includes a second gas concentration sensor configured to detect a second gas concentration and generate a corresponding second signal. In addition, the system includes a controller in communication with the first and second gas concentration sensors. Based on the first and second signals, the controller is configured to provide an out-of-tolerance gas concentration sensor warning.

In another aspect, the present disclosure is directed to a method of detecting when a gas concentration sensor has drifted or failed. The method includes detecting a first gas concentration with a first gas concentration sensor. Additionally, the method includes detecting a second gas concentration with a second gas concentration sensor. The method also includes calculating a difference between the detected first and second gas concentrations. In addition, the method includes providing an out-of-tolerance gas concentration sensor warning based on the calculated difference between the detected first and second gas concentrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed combustion engine;

FIG. 2 is a diagrammatic illustration of another exemplary disclosed combustion engine;

FIG. 3 is a diagrammatic illustration of yet another exemplary disclosed combustion engine; and

FIG. 4 is a flow chart describing an exemplary method of operating the exemplary combustion engines of FIGS. 1-3.

DETAILED DESCRIPTION

FIG. 1 illustrates a combustion engine 10, which may be utilized by various types of machines such as, for example, fixed or mobile machines that perform some type of operation associated with an industry such as mining, construction, farming, transportation, power generation, tree harvesting, forestry, or another industry known in the art. Combustion engine 10 may be an internal combustion engine, such as, for example, a diesel engine, a gasoline engine, or a natural gas engine. Combustion engine 10 may alternatively be another power source such as a furnace.

Operation of combustion engine 10 may produce power and a flow of exhaust. In particular, a controller 12 of combustion engine 10 may operate an air/fuel supply system 14 to supply an air/fuel mixture to a combustion chamber 16 of combustion engine 10. The air/fuel mixture may be combusted within combustion chamber 16, thereby producing power and the flow of exhaust. The flow of exhaust may include several chemicals such as, for example, carbon monoxide, carbon dioxide, NO_(x), ammonia, aldehyde(s), soot, oxygen, nitrogen, sulfur, water vapor, and/or hydrocarbons such as hydrogen and methane.

Some of the chemicals may be subject to emission standards (i.e. subject to minimum and/or maximum allowable emission concentrations). Therefore, the flow of exhaust may be directed to an exhaust system 18, which may directly and/or indirectly modify concentrations of the chemicals. For example, exhaust system 18 may include a catalytic converter 20 (referring to FIG. 2), a filter (not shown), an SCR system (not shown) and/or another exhaust treatment device to directly modify concentrations of the chemicals. And, exhaust system 18 may include a gas concentration sensor 22, and/or another sensing device known in the art to indirectly modify concentrations of the chemicals. This indirect modification may be by way of controller 12, which may adjust the air/fuel ratio of the air/fuel mixture (hereafter “the air/fuel ratio”) based on a first gas concentration detected by gas concentration sensor 22. It is contemplated, however, that gas concentration sensor 22 may become faulty or miscalibrated (hereafter “broken”). Therefore, exhaust system 18 may include another gas concentration sensor 24, which controller 12 may use to verify the gas concentrations detected by gas concentration sensor 22. If gas concentration sensor 22 is out-of-tolerance (i.e. broken, and therefore providing inaccurate gas concentrations to controller 12), controller 12 may alternatively adjust the air/fuel ratio based on a second gas concentration detected by gas concentration sensor 24. Controller 12 may also activate a warning device 25 if gas concentration sensor 22 is out-of-tolerance. For example, warning device 25 may embody a warning lamp; alarm; horn; head-up display; odorant or tissue-irritating substance dispenser; transmission means; or other device operable to provide an out-of-tolerance gas concentration sensor 22 warning to an individual located nearby or remote to combustion engine 10. Alternatively, it is contemplated that controller 12 may use gas concentration sensor 22 to verify the second gas concentration detected by gas concentration sensor 24.

Controller 12 may embody, for example, an engine control module, and may include means for monitoring, recording, storing, indexing, processing, and/or communicating information. These means may include, for example, a memory, one or more data storage devices, a central processing unit, and/or another component that may be used to run the disclosed applications. In particular, controller 12 may include a clock 26 to measure time and generate a corresponding time signal. Furthermore, although aspects of the present disclosure may be described generally as being stored in memory, one skilled in the art will appreciate that these aspects can be stored on or read from different types of computer program products or computer-readable media such as computer chips and secondary storage devices, including hard disks, floppy disks, optical media, CD-ROM, or other forms of RAM or ROM.

As previously discussed, controller 12 may operate air/fuel supply system 14 to adjust the air/fuel ratio supplied to combustion chamber 16. Air/fuel supply system 14 may be in fluid communication with and located upstream of combustion chamber 16. Air/fuel supply system 14 may include an air intake system 28 and a fuel system 30, both operable by controller 12. Air intake system 28 may include various components and/or systems for adjusting the amount of air supplied to combustion chamber 16, including, but not limited to, throttles, variable-output superchargers, and variable-valve-timing systems. Fuel system 30 may include various components for adjusting the amount of fuel supplied to combustion chamber 16, including, but not limited to, fuel injectors, variable-output pumps, valves, and carburetors.

Combustion chamber 16 may be formed out of a cylinder, a piston, and a cylinder head. It is contemplated that combustion engine 10 may include one or more combustion chambers 16, and that combustion chambers 16 may be disposed in an “in-line” configuration, a “V” configuration, or another suitable configuration. Alternatively, combustion chamber 16 may be formed out of a rotor, which is roughly triangular, and a housing (applicable to rotary engines). In yet another alternative, combustion chamber 16 may be formed out of other components known in the art and may embody, for example, a firebox (applicable to a furnace or boiler) or a flame holder (applicable to a jet engine). As previously discussed, combustion within combustion chamber 16 may produce a flow of exhaust, which may be directed to exhaust system 18, located downstream of combustion chamber 16.

Exhaust system 18 may include a flow line 32, which may further direct the flow of exhaust along a path 34. Path 34 may lead to one or more exhaust system 18 components. For example, flow line 32 may embody a pipe, a tube, a conduit, or another exhaust-carrying structure known in the art. It is contemplated that flow line 32 may branch into two or more subsidiary flow lines 36, thereby creating alternative paths 38. For example, flow line 32 may branch into subsidiary flow lines 36 a and 36 b, thereby creating alternative paths 38 a and 38 b, respectively. Furthermore, it is contemplated that two or more subsidiary flow lines 36 may rejoin into flow line 32, thereby rejoining alternative paths 38 into path 34. For example, subsidiary flow lines 36 a and 36 b may rejoin into flow line 32, thereby rejoining alternative paths 38 a and 38 b into path 34. If alternative paths 38 rejoin into path 34, it is contemplated that one alternative path 38 may be designated a normal path 40, and another alternative path 38 may be designated a bypass path 42 (hereafter “bypass 42”). These designations may correspond to an average level of exhaust flow (hereafter “exhaust flow”) along each alternative path 38. For example, the exhaust flow along normal path 40 may be greater than the exhaust flow along bypass 42.

Controller 12 may use a valve 44, situated along bypass 42, to adjust the exhaust flow along bypass 42. Controller 12 may open valve 44 to upwardly adjust the exhaust flow along bypass 42. And, controller 12 may close valve 44 to downwardly adjust the exhaust flow along bypass 42. It is contemplated that controller 12 may use another valve 46, also situated along bypass 42, to further adjust the exhaust flow along bypass 42. Controller 12 may open valve 46 to adjust upwardly the exhaust flow along bypass 42. And, controller 12 may close valve 46 to adjust downwardly the exhaust flow along bypass 42. It is further contemplated that controller 12 may use yet another valve 48, situated between bypass 42 and an atmospheric vent 50, to further adjust the exhaust flow along bypass 42. Atmospheric vent 50 may embody a fluid connection between bypass 42 and an atmosphere (i.e. a fluid external to exhaust system 18). When controller 12 opens valve 48, this fluid connection may serve to adjust the exhaust flow along bypass 42 downward and an atmospheric air flow along bypass 42 upward. And, when controller 12 closes valve 48, the fluid connection may serve to adjust the exhaust flow along bypass 42 upward and the atmospheric air flow along bypass 42 downward.

As previously discussed, exhaust system 18 may include gas concentration sensors 22 and 24. Each of gas concentration sensors 22 and 24 may embody a device that detects a gas concentration and generates a corresponding signal that may be communicated to controller 12. For example, each of gas concentration sensors 22 and 24 may embody a NO_(x) sensor. It is contemplated that the arrangement of exhaust system 18 components, including gas concentration sensors 22 and 24, may vary depending on application.

As illustrated in FIG. 1, gas concentration sensor 22 may be located along path 34. Bypass 42 may be located downstream of gas concentration sensor 22. Valve 44 may be located along bypass 42 and upstream of gas concentration sensor 24. And, valve 46 may be located along bypass 42 and downstream of gas concentration sensor 24. Valve 48 may be located downstream of valve 44 and upstream of valve 46.

Alternatively, as illustrated in FIG. 2, gas concentration sensor 22 may be located along path 34. Catalytic converter 20 may be located along path 34 and downstream of gas concentration sensor 22. Bypass 42 may be located downstream of catalytic converter 20. Valve 44 may be located along bypass 42 and upstream of gas concentration sensor 24. And, valve 46 may be located along bypass 42 and downstream of gas concentration sensor 24. Valve 48 may be located downstream of valve 44 and upstream of valve 46.

In yet another alternative, as illustrated in FIG. 3, exhaust system 18 may include a bypass 42 a and a bypass 42 b. Valve 44 a may be located along bypass 42 a and upstream of gas concentration sensor 22. And, valve 46 a may be located along bypass 42 a and downstream of gas concentration sensor 22. Valve 48 a may be located downstream of valve 44 a and upstream of valve 46 a. Bypass 42 b may be located downstream of bypass 42 a. Valve 44 b may be located along bypass 42 b and upstream of gas concentration sensor 24. And, valve 46 b may be located along bypass 42 b and downstream of gas concentration sensor 24. Valve 48 b may be located downstream of valve 44 b and upstream of valve 46 b.

FIG. 4 illustrates an exemplary method of operating the disclosed system. FIG. 4 will be discussed in the following section to further illustrate the disclosed system and its operation.

INDUSTRIAL APPLICABILITY

The disclosed system may be applicable to combustion engines, which may be subject to emission standards. The system may determine whether the combustion engine is operating properly. In particular, the system may detect when a gas concentration sensor has drifted or failed. Operation of the system will now be described.

As illustrated in FIG. 4, the disclosed system, and more specifically, controller 12 (referring to FIGS. 1-3), may continuously or intermittently adjust the/air fuel ratio supplied to combustion chamber 16 (referring to FIGS. 1-3) based on the detections of gas concentration sensor 22 (referring to FIGS. 1-3) (step 100). At certain time intervals and without discontinuing step 100 (i.e. while still continuously or intermittently adjusting the air/fuel ratio), controller 12 may test gas concentration sensor 22 (step 110). If gas concentration sensor 22 is not broken, controller 12 may return to step 100 without further action. But, if gas concentration sensor 22 is broken, controller 12 may discontinue step 100 (step 120), and provide an out-of-tolerance warning (step 140). Additionally, controller 12 may continuously or intermittently adjust the air/fuel ratio supplied to combustion chamber 16 based on the detections of gas concentration sensor 24 (referring to FIGS. 1-3) (step 150).

The adjustment of step 100 may include sub-steps. In particular, step 100 may include the sub-step of adjusting the air/fuel ratio based on the detections of gas concentration sensor 22 (sub-step 160). Step 100 may also include the sub-step of determining whether to test gas concentration sensor 22 (sub-step 170). Additionally, step 100 may include the sub-step of pausing, thereby causing the adjustment of sub-step 160 to be intermittent (sub-step 180). Alternatively, step 100 may not include sub-step 180, and the adjustment of sub-step 160 may be continuous.

At sub-step 160, it is contemplated that controller 12 may communicate with air/fuel supply system 14 to adjust the air/fuel ratio supplied to combustion chamber 16 based on the detection of gas concentration sensor 22. For example, controller 12 may adjust the air/fuel ratio until gas concentration sensor 22 senses NO_(x) at a concentration within five parts-per-million of the nominal.

After or while adjusting the air/fuel ratio, controller 12 may proceed to sub-step 170 and determine whether to test gas concentration sensor 22. Controller 12 may base this determination on the signal of clock 26. In particular, controller 12 may test gas concentration sensor 22 (i.e. proceed to step 110) at predetermined time intervals. Alternatively, controller 12 may test gas concentration sensor 22 based on measured parameters such as, for example, the detections of gas concentration sensor 22. Specifically, if the detections of gas concentration sensor 22 vary substantially and unexpectedly over time, controller 12 may test gas concentration sensor 22.

Next, controller 12 may pause (sub-step 180). This pause may temporarily prevent the adjustment of sub-step 160, thereby allowing for the temporary disablement of gas concentration sensor 22. When gas concentration sensor 22 is disabled, it may be shielded from the flow of exhaust. For example, controller 12 may shield gas concentration sensor 22 from the flow of exhaust by opening valve 48 a, and closing valves 44 a and 46 a (referring to FIG. 3). Based on the signal of clock 26, after a predetermined amount of time, controller 12 may unshield and reenable gas concentration sensor 22, and proceed back to sub-step 160. Alternatively, step 100 may not include sub-step 180, and the adjustment of sub-step 160 may be continuous. This continuous adjustment may be necessitated by certain emission standards such as, for example, those applicable to gasoline engines.

The testing of step 110 may include sub-steps. In particular, step 110 may include the sub-step of adjusting the exhaust and atmospheric air flows along bypass 42 to unshield gas concentration sensor 24 from the flow of exhaust (sub-step 190). Step 110 may also include the sub-step of pausing, thereby allowing the adjusted flows to increase the temperature of gas concentration sensor 24 (sub-step 200). Additionally, step 110 may include the sub-step of detecting and calculating an average of a first gas concentration, and detecting and calculating an average of a second gas concentration (sub-step 210). Step 110 may also include the sub-step of calculating the difference between the calculated averages of sub-step 210 (sub-step 220). Step 110 may further include the sub-step of adjusting the exhaust and atmospheric air flows along bypass 42 to shield gas concentration sensor 24 from the flow of exhaust (sub-step 230). In addition, step 110 may include the sub-step of determining whether the difference of sub-step 220 is greater than an allowable tolerance (sub-step 240). If this difference is greater than the allowable tolerance, gas concentration sensor 22 may be broken, and controller 12 may proceed to step 120. But, if the difference is not greater than the allowable tolerance, gas concentration sensor 22 may not be broken, and controller 12 may return to step 100.

The adjustment of sub-step 190 may also include sub-steps. In particular, sub-step 190 may include the sub-step of closing valve 48 (referring to FIGS. 1 and 3, and referring to valve 48 a in FIG. 2) (sub-step 250). Closing valve 48 may adjust downwardly the atmospheric air flow along bypass 42 (referring to FIGS. 1 and 3, and referring to bypass 42 a in FIG. 2). Sub-step 190 may also include the sub-steps of opening valve 44 (referring to FIGS. 1 and 3, and referring to valve 44 a in FIG. 2) (sub-step 260) and opening valve 46 (referring to FIGS. 1 and 3, and referring to valve 46 a in FIG. 2) (sub-step 270). Opening valve 44 may adjust upwardly the exhaust flow along bypass 42. And, opening valve 46 may also adjust upwardly the exhaust flow along bypass 42. It is contemplated that controller 12 may execute sub-step 250 before proceeding to sub-steps 260 and 270, thereby reducing communication of exhaust gasses to the atmosphere. Alternatively, controller 12 may execute sub-steps 250, 260, and 270 concurrently.

Next, controller 12 may pause (sub-step 200). This pause may temporarily prevent controller 12 from receiving signals regarding gas concentration sensor 24 detections. This pause may be necessary to allow gas concentration sensor 24 to reach its operational temperature. In particular, this pause may allow the exhaust flow to increase the temperature of gas concentration sensor 24.

After sub-step 200, controller 12 may detect and calculate an average of a first gas concentration, and detect and calculate an average of a second gas concentration (sub-step 210). Sub-step 210 may include the sub-step of detecting concentrations of the first and second gas (sub-step 280). It is contemplated that controller 12 may concurrently receive signals from gas concentration sensors 22 and 24 regarding detections of the concentrations of the first and second gasses, respectively. These signals may be received for a predetermined amount of time. Therefore, controller 12 may receive a certain number of detections from each of gas concentration sensors 22 and 24. Controller 12 may calculate a moving average of these detections as they are received. Alternatively, sub-step 210 may include the sub-step of calculating the averages of the first and second detected gas concentrations, respectively, after the predetermined amount of time has expired (i.e. when controller 12 is no longer receiving detections from gas concentration sensors 22 and 24) (sub-step 290). For example, these averages may be arithmetic means.

Next, controller 12 may calculate the difference between the calculated averages of sub-step 210 (sub-step 220). It is contemplated, however, that these averages may not be directly comparable if gas concentration sensors 22 and 24 are separated by another exhaust system 18 component that alters the gas. For example, if catalytic converter 20 is located downstream of gas concentration sensor 22 and upstream of gas concentration sensor 24 (referring to FIG. 2), the detections of gas concentration sensor 22 may not be directly comparable to those of gas concentration sensor 24 (i.e. the average of the first gas concentration may not be directly comparable to the average of the second gas concentration). It is contemplated that the averages may be compared after adjusting the average of the second gas concentration. Controller 12 may adjust the average of the second gas concentration based on a known affect of the intermediary exhaust system 18 component. Controller 12 may then subtract the adjusted average of the second gas concentration from the average of the first gas concentration. Alternatively, controller 12 may subtract the average of the first gas concentration from the adjusted average of the second gas concentration. Controller 12 may then determine the absolute value of the result of either of the above subtractions. This absolute value may be the difference between the calculated averages of sub-step 210.

Before or after sub-step 220, controller 12 may again adjust the exhaust and atmospheric air flows along bypass 42 (sub-step 230). This adjustment of the flows may include sub-steps. In particular, sub-step 230 may include the sub-step of opening valve 48 (sub-step 300). Opening valve 48 may adjust upwardly the atmospheric air flow along bypass 42. Sub-step 230 may also include the sub-steps of closing valve 44 (sub-step 310) and closing valve 46 (sub-step 320). Closing valve 44 may adjust downwardly the exhaust flow along bypass 42. And, closing valve 46 may also adjust downwardly the exhaust flow along bypass 42. It is contemplated that controller 12 may execute sub-steps 310 and 320 before proceeding to sub-steps 300, thereby reducing communication of exhaust gasses to the atmosphere. Alternatively, controller 12 may execute sub-steps 300, 310, and 320 concurrently.

After sub-step 220, controller 12 may determine whether the difference between the calculated averages of sub-step 210 (i.e. the difference of sub-step 220) is greater than an allowable tolerance. The allowable tolerance may be dictated by the emission standards applicable to combustion engine 10. In particular, controller 12 may subtract the difference of sub-step 220 from the allowable tolerance. If the result of this subtraction is a non-negative number (i.e. the difference between the calculated averages of sub-step 210 is not greater than the allowable tolerance), controller 12 may determine that gas concentration sensor 22 is not broken. Controller 12 may then proceed to step 100. On the other hand, if the result of the subtraction is a negative number, controller 12 may determine that gas concentration sensor 22 is broken. Controller 12 may then proceed to step 120.

At step 120, controller 12 may discontinue step 100 (i.e. discontinue adjustment of the air/fuel ratio based on gas concentration sensor 22 detections). Controller 12 may then provide an out-of-tolerance warning (step 140). In particular, controller 12 may activate warning device 25.

Next, controller 12 may continuously or intermittently adjust the/air fuel ratio based on the detections of gas concentration sensor 24 (step 150). Similar to step 100, the adjustment may include sub-steps. In particular, step 150 may include the sub-step of adjusting the air/fuel ratio based on the detections of gas concentration sensor 24 (sub-step 330). Additionally, step 150 may include the sub-step of pausing, thereby causing the adjustment of sub-step 330 to be intermittent (sub-step 340). Alternatively, step 150 may not include sub-step 340, and the adjustment of sub-step 330 may be continuous.

It is contemplated that adjusting the air/fuel ratio based on the detections of gas concentration sensor 24 may reduce the economic impact of a failure of gas concentration sensor 22. Specifically, after gas concentration sensor 22 has failed, combustion engine 10 may remain in compliance with emission standards while continuing to produce power and an exhaust flow. Therefore, gas concentration sensor 22 may be replaced when it is convenient to do so, for example, at a standard maintenance interval. Thus, combustion engine 10 maintenance costs may be reduced by reducing the need for maintenance between standard maintenance intervals. Furthermore, shielding gas concentration sensors 22 and/or 24 from the exhaust flow may increase their respective functional lives. Thus, combustion engine 10 maintenance costs may be further reduced.

It is further contemplated that a detection of a miscalibrated gas concentration sensor 22 may further decrease combustion engine 10 maintenance costs. Specifically, where reliable testing and calibration equipment is unavailable, it need not be assumed that gas concentration sensor 22 has failed. Instead, as discussed above, combustion engine 10 may continue to produce power and a flow of exhaust while remaining in compliance with emission standards. Therefore, gas concentration sensor 22 need not be immediately replaced, but may instead be recalibrated when it is convenient to do so. Thus, combustion engine 10 maintenance costs may be further reduced by reducing the need for replacement parts.

It will be apparent to those skilled in the art that various modifications and variations can be made to the method and system of the present disclosure. Other embodiments of the method and system will be apparent to those skilled in the art from consideration of the specification and practice of the method and system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents. 

1. A drift and failure detection system for a gas concentration sensor, comprising: a first gas concentration sensor configured to detect a first gas concentration and generate a corresponding first signal; a second gas concentration sensor configured to detect a second gas concentration and generate a corresponding second signal; and a controller in communication with the first and second gas concentration sensors, and configured to provide an out-of-tolerance gas concentration sensor warning based on the first and second signals.
 2. The drift and failure detection system of claim 1, further including an exhaust system, wherein the first and second gas concentration sensors are located within the exhaust system.
 3. The drift and failure detection system of claim 2, wherein the exhaust system includes a first bypass, and the second gas concentration sensor is located within the first bypass.
 4. The drift and failure detection system of claim 3, wherein: the controller includes a clock configured to measure time and generate a corresponding time signal; the first bypass includes at least one valve to adjust a flow; and the controller is in further communication with the at least one valve and configured to control the at least one valve based upon at least one of the first, second, and clock signals.
 5. The drift and failure detection system of claim 4, wherein the flow is an exhaust flow.
 6. The drift and failure detection system of claim 3, wherein the exhaust system includes a second bypass, and the first gas concentration sensor is located within the second bypass.
 7. The drift and failure detection system of claim 1, further including an air/fuel supply system, wherein the controller is in further communication with the air/fuel supply system and configured to adjust an air/fuel ratio based on at least one of the first and second signals.
 8. The drift and failure detection system of claim 1, further including a warning device, wherein the controller is in further communication with the warning device and configured to activate the warning device based on the first and second signals.
 9. A method of detecting when a gas concentration sensor has drifted or failed, comprising: detecting a first gas concentration with a first gas concentration sensor; detecting a second gas concentration with a second gas concentration sensor; calculating a difference between the detected first and second gas concentrations; and providing an out-of-tolerance gas concentration sensor warning based on the calculated difference between the detected first and second gas concentrations.
 10. The method of claim 9, further including measuring a time.
 11. The method of claim 10, further including adjusting a flow based on the measured time.
 12. The method of claim 11, wherein the flow is an exhaust flow.
 13. The method of claim 9, wherein calculating the difference between the detected first and second gas concentrations includes: calculating an average detected first gas concentration; calculating an average detected second gas concentration; and comparing the average detected first gas concentration and the average detected second gas concentration.
 14. The method of claim 9, further including adjusting an air/fuel ratio based on at least one of the detected first and second gas concentrations.
 15. The method of claim 9, further including activating a warning device based on the calculated difference between the detected first and second gas concentrations.
 16. A combustion engine, comprising: an air/fuel supply system; a combustion chamber located downstream of the air/fuel supply system; an exhaust system located downstream of the combustion chamber, the exhaust system including: a first gas concentration sensor configured to detect a first gas concentration and generate a corresponding first signal; and a second gas concentration sensor configured to detect a second gas concentration and generate a corresponding second signal; and an engine control module in communication with the first and second gas concentration sensors, and the air/fuel supply system, and configured to: provide an out-of-tolerance gas concentration sensor warning based on the first and second signals; and adjust an air/fuel ratio based on at least one of the first and second signals.
 17. The combustion engine of claim 16, wherein the exhaust system further includes a first bypass, and the second gas concentration sensor is located within the first bypass.
 18. The combustion engine of claim 17, wherein: the engine control module includes a clock configured to measure time and generate a corresponding time signal; the first bypass includes at least one valve to adjust a flow; and the engine control module is in further communication with the at least one valve and configured to control the at least one valve based on at least one of the first, second, and clock signals.
 19. The combustion engine of claim 17, wherein the exhaust system further includes a second bypass, and the first gas concentration sensor is located within the second bypass.
 20. The combustion engine of claim 16, wherein: the first gas concentration sensor is a NO_(x) concentration sensor; and the second gas concentration sensor is a NO_(x) concentration sensor. 